Vinyl alcohol polymers (hereinafter, may be also referred to as “PVA(s)”) have superior film forming properties, interface characteristics and strength characteristics as a few water soluble crystalline macromolecules. Therefore, PVAs have been playing an important role as basic materials of thickeners, coating materials for papers, adhesives, fiber processing agents, binders, emulsion stabilizers, films and fibers, and the like.
Furthermore, in order to improve certain performances of PVAs, a variety of modified PVAs have been developed by way of controlling crystallinity as well as introduction of a functional group, etc. Among the modified PVAs, alkyl-modified PVAs having an alkyl group introduced thereto are known to present an alkyl group (hydrophobic group) interaction in an aqueous solvent, thereby increasing thickening properties (see Japanese Unexamined Patent Application, Publication No. S55-47256). The alkyl-modified PVAs exhibit more superior thickening properties as the content of alkyl groups increases; however, a too high content of the alkyl groups leads to inferior water solubility. Therefore, in attempts to increase water solubility and the like, alkyl-modified PVAs having an ionic functional group introduced thereto were proposed (see Japanese Unexamined Patent Application, Publication Nos. 558-15055, S59-78963 and H8-60137). In addition, for the purpose of increasing performances as an emulsion stabilizer, a modified PVA similarly having an alkyl group and an ionic functional group (carboxyl group) was proposed (see Japanese Unexamined Patent Application, Publication No. H8-281092).
However, when an ionic functional group is introduced into an alkyl-modified PVA, a disadvantage may occur in the case in which the alkyl-modified PVA coexists with a substance having opposite ionicity, that binding with the substance results in deterioration of the thickening properties and storage stability. In addition, when used in, for example, films, adhesives, etc., the alkyl-modified PVA having an ionic functional group introduced thereto may have insufficient water resistance due to the presence of the ionic functional group, and when produced into films, for example, a disadvantage of deterioration of water repellency on the surface may also occur.
Furthermore, since PVAs are water soluble as described above, the case in which drying is carried out at particularly low temperatures is accompanied by a problem of insufficient water resistance of the coating film obtained. In order to solve this problem, a variety of improvements of PVAs or compositions thereof have been investigated. As a method for improving the water resistance, for example, a method in which a PVA is crosslinked with glyoxal, glutaraldehyde, a dialdehyde starch, a water soluble epoxy compound, a methylol compound or the like has been known. However, for making the coating film sufficiently water resistant by this method, it is necessary to carry out a heat treatment at temperatures as high as no less than 100° C., particularly no less than 120° C. for a long period of time.
In addition, according to a method in which a PVA-coating film is made water resistant by drying at low temperatures, a technique of preparing an aqueous PVA solution to give a strongly acidic condition such as for example, a pH of no greater than 2 has been known. However, in this case, a disadvantage of impaired viscosity stability of the aqueous PVA solution to result in gelation during use, and the like may occur, and also it would not be possible to attain sufficient water resistance of the coating film obtained.
Moreover, a method in which a carboxyl group-containing PVA is crosslinked with a polyamide epichlorohydrin resin; a method in which an acetoacetyl group-containing PVA is crosslinked with a polyvalent aldehyde compound such as glyoxal; a method in which crosslinking is permitted with a polyfunctional epoxy compound, a polyfunctional carboxy compound or a boron compound (see Japanese Unexamined Patent Application, Publication No. 2010-111969); and the like have been also investigated. However, when the crosslinking agent used in each of these methods is employed, a disadvantage of impaired viscosity stability of the aqueous PVA solution (coating material), etc., may occur.
On the other hand, adhesives containing a PVA as a principal component are inexpensive and have superior adhesiveness, and have been used for adhering paper boards, corrugated cardboards, paper tubes, sliding doors (i.e., fusuma), wall papers, wood building materials, and the like. In addition, adhesives containing a mixture of various types of emulsion and a PVA have been used as adhesives for woodworking, for fiber processing and for papers, and the like. Accordingly, the aqueous adhesives containing a PVA (i.e., PVA type aqueous adhesives) have been used for a broad range of intended usage. However, also with regard to PVA type aqueous adhesives, in order to meet an increase in coating process speed and the like in recent years, further improvement of adhesiveness that includes initial adhesiveness, as well as storage stability (viscosity stability and dispersion stability), and the like has been desired.
Under such circumstances, (1) an adhesive containing a PVA, a clay and a water soluble boron compound as principal components (see Japanese Unexamined Patent Application, Publication No. S62-195070), (2) a PVA type adhesive containing a certain metal salt (see Japanese Unexamined Patent Application, Publication No. H4-239085), (3) an adhesive containing a modified polyvinyl alcohol having an ethylene unit in an amount of 1 to 20 mol %, a starch and a saccharide (see Japanese Unexamined Patent Application, Publication No. H11-21530), and (4) an adhesive containing a vinyl alcohol polymer that includes 1.8 to 3.5 mol % 1,2-glycol bond in the molecule and has a degree of saponification of no less than 90 mol %, and an inorganic filler (see Japanese Unexamined Patent Application, Publication No. 2001-164219) were proposed.
The adhesive (1) can improve initial adhesiveness and has been industrially used broadly in manufacture of corrugated cardboards, and the like. However, use of a boron compound by which influences on the environment are concerned has been gradually restricted in recent years, and thus an alternative thereof has been strongly desired. In addition, a large number of attempts to improve initial adhesiveness similarly by using a candidate compound of a crosslinking agent of a PVA (for example, urea-formalin based resin, etc.) have been also made. However, there are some cases in which safety of the compound used as a crosslinking agent is substantially concerned, and also viscosity stability of the composition is not yet sufficient in many cases under current circumstances.
In addition, although the adhesive (2) has improved initial adhesiveness, its storage stability is problematic, and is thus not industrially satisfactory enough.
Furthermore, the adhesives (3) and (4) also have improved adhesiveness and storage stability to some extent; however, they do not sufficiently meet demands for an increase in coating process speed in recent years.
Additionally, these PVA type adhesives may be used as a mixture with various types of emulsion as described above for the purpose of improving adhesiveness and the like by increasing the concentration of the solid content, etc., improvement of the aforementioned performances (i.e., adhesiveness, storage stability, and the like) have been still desired for these adhesives.